Microwave transmission line

ABSTRACT

A microwave transmission structure consisting of a hollow conductive waveguide with at least one film conductor supported on a dielectric substrate within the guide to provide a conductive surface projecting inwardly from the guide wall in the manner of a ridge in ridgeguide.

United States Patent 11?] Meier MICROWAVE TRANSMISSION LINE [75]Inventor:

' .[73] Assignee: Cutler-Hammer, Inc., Milwaukee,

Paul J. Meier, Westbury, N.Y.

Wis.

22 Filed: May 18,1973

I (211 Appl. No.: 361,634

[52] 11.5. CI. 333/95 R, 333/98 R, 329/161 [51] Int. Cl. 1101p 3/12,I-IOlp 1/00 [58] Field 01 Search.... 333/95 R, 98 R, 31 A, 84 R,

[56] References Cited UNITED STATES PATENTS 2,155,508 4/1939 Schelkunoff333/95 R 2,591,329 4/1952 Zaleski 333/31 A 2,921,263 H1960. Jaffe 333/98R 3,649,935 3/1972 Low 333/98 R 3,732,508 5/1973 Ito et al.... 333/84 R3,760,302 9/1973 Cohn 333/84 R OTHER PUBLICATIONS Eaves et al., Modes onShielded Slot Lines, Arch.

, III/III/I/ [111 3,825,863 [4 1 July 23, 1974 Elekt Ubertragung, Vol.24, 1970, pp. 389-394.

Minor, J. C., Propagation in Shielded Microslot on Ferrite Substrate,"Electronics Lett. Vol. 7, 1971, pp. 502-504.

Fox, A. 6., An Adjustable Wave-Guide Phase Changer Pro. IRE, 1947, pp.1,489, 1,495.

Primary Exarniner-Archie R. Borchelt Assistant ExaminerWm. H. PunterAttorney, Agent, or Firm-Henry Huff; Kevin Redmond [57] ABSTRACT Amicrowave transmission structure consisting of a hollow conductivewaveguide with at least one film conductor supported on a dielectricsubstrate within the guide to provide a conductive surface projectinginwardly from the guide wall in the manner of a ridge in ridgeguide.

7 Claims, 5 Drawing Figures 1 MICROWAVE TRANSMISSION LINE BACKGROUND .1-F eldef the Inventi n- The invention pertains to microwave transmissionlines comprising a dielectric substrate clad with a film conductorenclosed by and cooperating with a waveguide. 1 v

2. Descriptionpf the Prior Art Many known arrangements of filmconductors on dielectric subs ates are used as microwave transmissionlines. These 1 nes, referred to herein as printed transmission lines,clude slot-line, dielectric sandwich line, inverted micro rip,'suspendedstrip line and coplanar line. The foregoing types of transmission linescan be used to produce low cost, intricate microwave circuits,compatible with discrete components at low microwave frequencies;however, they are not entirely satisfactory at high microwavefrequencies for various reasons including high fabrication costs, highloss, critical tolerance requirement, fragile substrates, thin conductorstrips, difficulty in mounting discrete components, and in obtaining'asimple transition toconventional waveguide. The principal object ofthis invention is to provide an improved type of transmission linehaving the desirable characteristics of printed transmission lineswithout the above mentioned inadequacies.

SUMMARY According to this invention, a printed transmission line analogof ridgeguide, called fin-line, is formed by providingconductive finswithin a hollow waveguide. The fins are comprised of conductive film ona dielectric substrate and cooperate with the waveguide in a' mannersimilar to that of the ridges in ridgeguide, increasing the separationbetween the first and second modes of propagation, and thereby providinga wider microstrip line width, easing the printing tolerances andfacilitating the installation of discrete components. Since there is noground plane, the critical spacing from line to ground plane occurringin most printed transmission lines is eliminated permitting the use of athicker and therefore stronger dielectric substrate.

Due to the elimination of the ground plane and the use of a waveguideenclosure, the E-fieldneed not be concentrated within the dielectricsubstrate, permitting a reduction of the dielectric loss. Radiation lossis eliminated by confining the E-field within the guide walls.

These reductions in line loss permits the fabrication of high Q circuitelements not previously achievable with conventional printedtransmission lines. Further performance details may be found in P. J.Meier, Two

I New Integrated-Circuit Media with Special Advantages posium Digest, p.221-223, May 22, 1972.

useful bandwidth than conventional waveguide. The a similarity of thisstructure to standard waveguide facili- DRAWINGS FIG. 1 is a perspectiveview of an embodiment of the invention showing the relative location ofthe waveguide, dielectric substrate, film conductors, matching sectionsand discrete components.

FIG. 2 is a graph of the unloaded Q as a function of the normalized gapfor the embodiments of FIG. 1 and FIG. 4.

FIG. 3 is a graph of the variation in equivalent dielectric constant asa function of the normalized gap for the embodiments of FIG. 1 and FIG.4.

FIG. 4 is a cross section of an embodiment of the in-' vention showinguseful design dimensions and a first method of introducing bias on thefilm conductors.

FIG. 5 is a cross section of an embodiment of the invention showing asecond method of introducing bias on the film conductors.

DESCRIPTION Referring to FIG. 1, a dielectric substrate 2, clad withfilm conductors 4 and 5 and supporting discrete circuit elements 6 and7, is mounted within waveguide l. Gap 3, normally located midway of thewaveguide, separates film conductors 4 and 5. Substrate 2 is preferablymounted in the center of the guide causing the maximum field strength toexist across gap 3. The edges of conductors 4 and 5 remote from the gap3 are electrically connected to the waveguide wall. In more complicatedconfigurations, the fin conductors may be dc isolated from the wallswhile still being connected at RF to permit the introduction of bias.

Referring to FIG. 2, the higher Q achievable 'with either waveguideconnected or waveguide isolated fins as compared to microstrip is shown.In this figure, ordinate axis 8 represents unloaded Q. Abscissa 9represents a normalized gap defined as the ratio of the gap width (d) toguide height (b). The Q of X-band microstrip is shown to be 250 bydotted line 12. Graphl0 shows the variation in Q when the fin conductorsare directly connected tothe waveguide wall. Graph 11 shows thevariation in Q when the fin conductors are dc isolated from thewaveguide walls. The data shown in FIGS. 2 and 3 was measured at X-bandusing a teflon-fibreglass substrate with a dielectric constant of 2.5 ina fin-line whose aspect ratio (b/a) was 0.45, and whose c/a ratio was0.07, where a is the internal guide width and c is the dielectricthickness.

Referring to FIG. 3, the equivalent dielectric constant, K, of thewaveguide is plotted as a function of normalized gap width. In thisfigure, ordinate 13 is K, while abscissa 14 is the normalized gap width(d/b). Graph 15 shows the variation in K, for dc isolated fins whilegraph 16 shows the variation for fin conductors connected to thewaveguide wall. It can be seen that a high value of K approaches that ofthe substrate material for isolated fins with narrow normalized gapwidths. The value of K drops to 1.25 for both connected and isolatedfins when the normalized gap width is 1.0. The value of K, at this pointis only slightly above that of air,

and represents the relatively small effect a dielectric substrate alonecan have on the effective dielectric constant of the guide.

Data available in S. Hopfer, The Design of Ridged- Waveguides, IRETrans, Vol. MTT-3, p. 20-29, Oct.

1955 can be used to facilitate the design of various practicalconfigurations of the subject invention. Thin, moderate dielectricconstant substrates have little effect on the field distributions, andtherefore the single mode bandwidth and attenuation can be estimateddirectly from ridgeguide data such as found in the above reference.

Lines constructed in accordance with the present invention possess anumber of the more desirable characteristics of printed transmissionlines and ridgeguide including large dimensions at higher microwavefrequencies, low loss, and ease of pattern variation. Line dimensionssignificantly larger than those of printed transmission lines aid ineasing the fabrication process, facilitating the addition of discretecomponents, and reducing the copper loss.

To produce a 50 ohm transmission line at 60 GHz in microstrip, theprinted line width must be less than 0.005 inches and the substratethickness must be less than 0.002 inches in order to prevent excessiveradiation with common substrate material. If an alumina substrate isused at the same frequency, both the printed line width and thedielectric substrate thickness must be less than 0.004 inches.

In either case, the printed line widths are sufficiently narrow to makeline fabrication and lead attachment difficult. The thin substrates areweak and costly to produce. On the other hand, a gap width of 0.01inches and a dielectric thickness of 0.02 inches can be used forfin-line at 60 GI-Iz, significantly alleviating the dimensional problemsencountered with printed transmission lines noted above.

The use of a film conductor clad substrate facilitates the fabricationof a variety of patterns to produce various line impedances, matchingsections and filter elements. Complicated patterns can be produceduniformly and in large volume by processes such as photoetching whilethe production of similar patterns in ridgeguide would require expensivemachining.

As noted previously, embodiments of the presentinvention are adaptableto hybrid integrated circuit components including active devicesrequiring bias. Although the film conductors can be directly connectedto the waveguide walls for passive devices, isolation of the filmconductors from each other at dc is necessary to permit bias to besupplied through the conductors to active devices. At the same time, theconductors must have continuity with the waveguide wall at RFfrequencies.

A way in which this may be accomplished is shown in FIG. 4. Waveguide 1is divided in two along the longitudinal axis at the center of thebroadwalls and a dc isolated fin-line structure is inserted into theguide. This structure is comprised of a first dielectric sheet 2, twofilm conductors 4 and 5, and second dielectric sheet 21. The filmconductors are located between the two dielectric sheets and thereforeare isolated from the waveguide walls at dc. Film conductors 4 and areseparated by gap 3 of width d. Bias is placed on film conductor 4through lead and on film conductor 5 through lead 19. RF continuitybetween the fins and the waveguide wall and between the two segments ofthe wall is obtained by using an RF choke formed by choosing theinternal length of upper flange 17 and lower flange 18 to be aquarter-wavelength in the dielectric medium.

A second method of introducing bias on the film conductors is shown inFIG. 5. In this configuration, the waveguide l is divided into twohalves along the longitudinal axis at the center of the broadwalls and afin- 5 line structure is inserted between the two halves. This fin-linestructure is comprised of a single dielectric substrate, clad on theupper left-hand side with film conductor 22 and on the lower right-handside with film conductor 5. Gap 3 is located between the conductors andone-quarter wavelength flanges 17 and 18 are used for mounting and RFcontinuity.

Each half of the waveguide is dc isolated from the other by thedielectric 2, and each of the film conductor makes contact with only onehalf of the waveguide. Bias can be introduced by connection to each filmconductor directly or through each half of the waveguide. The latterarrangement is shown in FIG. 5, where the bias is supplied to the filmconductors through the waveguide from leads 23 and 24. An active device25 is located in a hole drilled through the dielectric substrateadjacent gap 3. This device receives bias through its connections tofilm conductors 5 and 22.

I claim:

1. A microwave transmission line comprising:

a. a hollow conductive waveguide adapted to operate in a mode having atransverse electric component,

a dielectric substrate in the form of a sheetof such thickness anddielectric constant that the electric field is distributed principallyin the space within the guide surrounding the dielectric, said substratesheet having an edge secured to the inside of the waveguide wall with asurface extending inwardly of the guide away from the wall in atransverse direction across the guide and in the longitudinal directionof the guide over a distance substantially greater than the width of theguide in said trans verse direction,

. a first conductor formed of film material supported on said substratesurface extending inwardly of the guide and terminating'in an edge awayfrom the waveguide wall, said first conductor being RF connected to thewaveguide wall, and

d. a second conductor RF connected to the waveguide wall within theguide and separated from said edge of the first conductor by anonconductive gap.

2. A microwave transmission line as recited in claim 1, wherein the gapwidth is different at different regions along the line to providerespective different impedance levels in said regions.

3. A transmission line as recited in claim 2, wherein said substrate isa planar sheet and said second conductor is a second film conductorsupported on said dielectric substrate surface.

4. A microwave transmission line as recited in claim 3, furthercomprising:

a. an RF choke including outwardly extending flanges to mount thedielectric substrate and maintain RF connection of the conductors to thewaveguide wall and RF continuity of the waveguide wall, said flangesbeing located along a longitudinal division in the waveguide wall,

b. a second dielectric substrate located between said film conductorsand said flanges to provide dc isolation of the film conductors fromeach other and flange being located along two longitudinal'divisions'made in oppositewalls of the guide which divide the guide in twosegments, direct connection being made by each film conductor to aseparate segment of the waveguide wall, whereby said film conductors canbe used as bias conductors.

7. A microwave transmission line as claimed in claim 1, wherein saidwaveguide is rectangular and said dielectric substrate is perpendicularto a broad wall near the center of the guide.

1. A microwave transmission line comprising: a. a hollow conductivewaveguide adapted to operate in a mode having a transverse electriccomponent, a dielectric substrate in the form of a sheet of suchthickness and dielectric constant that the electric field is distributedprincipally in the space within the guide surrounding the dielectric,said substrate sheet having an edge secured to the inside of thewaveguide wall with a surface extending inwardly of the guide away fromthe wall in a transverse direction across the guide and in thelongitudinal direction of the guide over a distance substantiallygreater than the width of the guide in said transverse direction, c. afirst conductor formed of film material supported on said substratesurface extending inwardly of the guide and terminating in an edge awayfrom the waveguide wall, said first conductor being RF connected to thewaveguide wall, and d. a second conductor RF connected to the waveguidewall within the guide and separated from said edge of the firstconductor by a nonconductive gap.
 2. A microwave transmission line asrecited in claim 1, wherein the gap width is different at differentregions along the line to provide respective different impedance levelsin said regions.
 3. A transmission line as recited in claim 2, whereinsaid substrate is a planar sheet and said second conductor is a secondfilm conductor supported on said dielectric substrate surface.
 4. Amicrowave transmission line as recited in claim 3, further comprising:a. an RF choke including outwardly extending flanges to mount thedielectric substrate and maintain RF connection of the conductors to thewaveguide wall and RF continuity of the waveguide wall, said flangesbeing located along a longitudinal division in the waveguide wall, b. asecond dielectric substrate located between said film conductors andsaid flanges to provide dc isolation of the film conductors from eachother and from the flanges, whereby said film conductors can be used asbias conductors.
 5. A microwave transmission line as recited in claim 2,wherein said substrate is a planar sheet, said second conductor is asecond film conductor supported on the opposite side of said substratefrom the first film conductor.
 6. A transmission line as recited inclaim 5, further comprising an RF choke including outwardly extendingflanges used to mount the dielectric and provide RF connection of thefilm conductors to the waveguide wall and RF continuity of the waveguidewall, said flange being located along two longitudinal divisions made inopposite walls of the guide which divide the guide in two segments,direct connection being made by each film conductor to a separatesegment of the waveguide wall, whereby said film conductors can be usedas bias conductors.
 7. A microwave transmission line as claimed in claim1, wherein said waveguide is rectangular and said dielectric substrateis perpendicular to a broad wall near the center of the guide.